Field of the Invention
This invention relates to a coal-water slurry, and more particularly it relates to a process for producing a coal-water slurry of the so-called good stability having a high coal concentration and a low viscosity with minimal settlings.
Recently coal has come to be actively used in place of petroleum mainly at thermal power stations. However, coal in the form of solid fuel is difficult to handle; hence large transport costs are required and there is a great influence on the cost of coal itself. Thus techniques by which coal is slurried to make it possible to handle coal in the form of fluid have been energetically developed. One of products thus developed is a mixture of heavy oil with coal (Coal and Oil Mixtures, hereinafter referred to as "COM"). In the case of COM, however, the ratio by weight of heavy oil to coal is about 1:1; thus COM cannot be regarded as a oil-free fuel and also its merit in respect of cost is small. Further, methacoal in the form of a mixture of methanol with coal also has a high cost; hence it has not yet been practically used.
On the other hand, CWM in the form of a mixture of coal with water (CWM: abbreviation of Coal-Water Mixtures) is sufficiently practical in respect of cost; hence it has recently been greatly noted. However, a problem raised in the combustion of CWM is the water content in CWM. As its combustion efficiency is concerned, naturally the lower the water content, the better the efficiency, and in the case of direct combustion, a water content of 30% or less is preferred. However, the lower the water content, the higher the viscosity of CWM; this raises a problem that when it is transported by way of pipeline or the like, the pressure loss increases.
Further, when CWM is practically used, a problem of storage is also raised. When CWM is stored in a usual tank, it is necessary for it to have a superior stability, but since CWM consists of coal particles and water, it is preferred to reduce their particle diameter, in order to inhibit coal particles from settling as much as possible. However, there is a tendency that when the particle diameter is reduced, the viscosity increases.
In order to overcome such drawbacks, it has been attempted to adjust the particle diameter distribution of coal particles to thereby prepare a CWM of the so-called good stability having a high coal concentration and a low viscosity with minimal settlings. However, coal particles are not completely spherical, and also the method of measuring the particle diameter of coal particles are various as follows: a method by means of sieves, a settling method represented by Andreasen Pipette, a method of analyzing the particle shapes by way of photographs of SEM (Scanning Electron Microscope) to calculate their representative diameter, etc. Thus, the definition of the particle diameter also varies depending on the measurement methods. This causes errors in adjusting the particle diameter distribution, and it becomes difficult to produce a CWM having a high coal concentration, a low viscosity and a good stability.
Now, the present inventors have considered that this problem might solved by adjusting the particle diameter distribution according to a method of measuring the particle diameter distribution regarded as most adequate, and have made extensive research. As a result, we have succeeded in obtaining the objective CWM having a high coal concentration, a low viscosity and a good stability.
The object of the present invention is to provide a process for producing a coal-water slurry having a high coal concentration, a low viscosity and a good stability.